1. Field of the Invention
The present invention relates to a method of manufacturing a mounting substrate on which monolithic ceramic capacitors are mounted and a mounting structure.
2. Description of the Related Art
Monolithic ceramic capacitors have been widely used in mobile terminals such as cellular phones and audio players and various electronic devices such as personal computers. Such a monolithic ceramic capacitor includes a multilayer body in which a plurality of dielectric ceramic sheets are stacked on top of one another. In the multilayer body, planar inner electrodes are disposed in such a manner as to face each other with at least one of the dielectric ceramic sheets interposed therebetween, and at least a pair of outer electrodes that are electrically connected to the inner electrodes are provided on surfaces of the multilayer body. Outer electrodes of such a monolithic ceramic capacitor are mounted on lands that are formed on a circuit board via a joining material such as solder.
In recent years, electronic devices have been rapidly decreasing in size and rapidly increasing in performance. Accordingly, the capacitance of monolithic ceramic capacitors has been increased. High dielectric constant type ceramics such as barium titanate, calcium titanate, and strontium titanate are used as dielectric ceramic materials of large-capacitance monolithic ceramic capacitors. These high dielectric constant type ceramics, upon application of a voltage (an electric field) thereto, mechanically deform due to piezoelectric and electrostrictive characteristics thereof, such deformation being so-called electric field-induced strain. Therefore, when an alternating-current voltage or a direct-current voltage on which an alternating current component is superimposed is applied to such a monolithic ceramic capacitor, mechanical deformation vibration is generated in a multilayer body of the monolithic ceramic capacitor.
When the vibration due to the electric field-induced strain is transferred to a circuit board, the circuit board vibrates. In the case where the frequency of the vibration sound generated due to the vibration of the circuit board is in the range of 20 Hz to 20,000 Hz, which defines the audio frequency range, the vibration sound is audible to the human ear as an uncomfortable sound (e.g., a screech or an acoustic noise).
Japanese Unexamined Patent Application Publication Nos. 2000-232030 and 2003-318057 describe that the vibration sound of a circuit board may be reduced by arranging monolithic ceramic capacitors at positions on the front and rear surfaces of the circuit board that substantially correspond to each other. Monolithic ceramic capacitors are arranged on the front and rear surfaces of a circuit board, so that the directions of the deformations transferred to the circuit board from these capacitors are opposite to each other, and thus, the vibrations of the circuit board cancel each other out.
However, in the methods of mounting monolithic ceramic capacitors on a circuit board described in Japanese Unexamined Patent Application Publication Nos. 2000-232030 and 2003-318057, there have been problems in that the vibration sound of a circuit board is not always effectively reduced. The vibration of a monolithic ceramic capacitor is to be transferred to a circuit board via a joining material such as solder. The inventors of the present invention have discovered that, in this case, the effect of reducing vibration sound may sometimes not be obtained depending on variation in the formation state of a fillet formed of a joining material.